1. Field of the Invention
The present invention relates to a piezoelectric resonator using the third harmonic of a thickness vertical vibration mode, and more particularly, the present invention relates to a piezoelectric resonator that effectively utilizes the resonance characteristics generated by the third harmonic while preventing the generation of a fundamental wave that produces spurious responses.
2. Description of the Related Art
Conventionally, an energy-trap type piezoelectric resonator has been widely used in piezoelectric oscillators and other devices. Japanese Unexamined Patent Application Publication No. 6-112756 discloses a piezoelectric resonator using the third harmonic of a thickness vertical vibration as a resonator which can be used in a higher frequency range.
As illustrated in FIG. 10, in a conventional piezoelectric resonator 51, an excitation electrode 53 is located at the central portion of the upper surface of a piezoelectric substrate 52 made of a piezoelectric ceramic. At the central portion of the lower surface of the piezoelectric substrate 52, another excitation electrode (not shown) is arranged so as to be opposed to the excitation electrode 53 with the piezoelectric substrate 52 disposed therebetween. These excitation electrodes disposed on both main surfaces define an energy-trap type piezoelectric resonance section utilizing a third harmonic of a vertical vibration mode.
On the upper surface of the piezoelectric substrate 52, a lead-out electrode 54 is connected to the excitation electrode 53, and a terminal electrode 55 is connected to the outside end of the lead-out electrode 54. Likewise, on the lower surface of the piezoelectric substrate 52, a lead-out electrode and terminal electrode are connected to the excitation electrode. The terminal electrode located on the lower surface is, however, disposed at the opposite end from the terminal electrode 55 located on the upper surface on the piezoelectric substrate 52.
Also, the piezoelectric resonator 51 is coated with a damping material 56 consisting of resin. The damping material 56 is disposed outside of the area between the excitation regions of the fundamental wave and the third harmonic of the thickness vertical vibration when the piezoelectric resonator 51 is driven. That is, in the piezoelectric resonator 51 utilizing a thickness vertical vibration mode, the excitation region of the fundamental wave is located outside that of the third harmonic. Therefore, applying the damping material 56 to the area outside of the area between the two excitation regions permits utilization of the third harmonic by damping only the fundamental wave and weakly damping the third harmonic.
In this case, the diameter of a circular opening 56a where the damping layer is not applied is six to twelve times as long as the wavelength of the third harmonic used.
In the piezoelectric resonator 51, however, the accuracy of applying and arranging the damping material 56 is not sufficient, and therefore, the piezoelectric resonator 51 cannot sufficiently damp the fundamental wave.
Also, since the damping material 56 is arranged to surround the excitation electrode 53, the piezoelectric resonator 51 must have a much larger size than the diameter of the opening 56a of the damping material 56. This makes it difficult to reduce the size of the piezoelectric resonator 51.
In addition, since the damping material 56 is arranged to surround the excitation electrode 53, there is a risk of a crack being generated in the piezoelectric substrate 52 when a thermal shock, an external force, or other stress is applied to the piezoelectric substrate 52.